well you researched the other problems (congrats! :). You need Option "UseFBDev" or Option "ProgramFPRegs" "No" in the Device Section. Either should do, but the first one only works with aty128fb.
What's the meaning of these options? Are they needed only for 8bit depth mode? Since I have an imac with Ati Rage 128 and I do not need any of them. Gianluca